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http://www.glossary.oilfield.slb.com/en/Terms/o/overpressure...

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differential pressure
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1. n. [Drilling]
In general, a measurement of fluid force per unit area (measured in units such as pounds per square in.) subtracted from a higher measurement of fluid force per unit area. This comparison could be made between pressures outside and inside a pipe, a pressure vessel, before and after an obstruction in a flow path, or simply between two points along any fluid path, such as two points along the inside of a pipe or across a packer.
See: packer
2. n. [Drilling]
The change in force per unit area measured before and after drilling fluid passes through small-diameter bit nozzles.
See: bit nozzle
3. n. [Drilling]
The change in force per unit area measured across various downhole tools such as measurements-while-drilling (MWD) tools, downhole turbines and mud motors.
See: measurements-while-drilling, mud motor
4. n. [Drilling]
The change in force per unit area between the reservoir pore pressure and the wellbore fluid pressure. If this measurement becomes negative in value (that is, the reservoir pressure exceeds the wellbore fluid pressure), then a flow of reservoir fluids into the wellbore can result.
See: kick, pore pressure, reservoir pressure
5. n. [Production Testing]
The difference between two pressure measurements. For production wells, the differential pressure is the difference between average reservoir pressure and bottomhole pressure, and for injection wells, it is the difference between injection pressure and average reservoir pressure.